Hollow cathode discharge tubes



Nov. l5, 1966 MAsAo suGAwARA ETAL 3,285,119

HOLLOW CATHODE DISCHARGE TUBES Filed April 8, 1964 In UEDTO@ ATTORNEY United States Patent This invention relates to hollow cathode discharge tubes, and more particularly to those for use in the generation of resonance lines in atomic absorption spectrophotometers.

, The primary object ofthe present invention is to provide a hollow cathode discharge tulbe adapted for effective generation of resonance lines of low-melting-point metals, and particularly of alkali metals.

Another object of the present invention is to provide a hollow cathode discharge tube of the kind specified which is easy to manufacture and has a long useful life, Iand which is specially useful for generation of resonance lines in atomic absorption spectrophotometers.

There are other objects and particularities of the present invention, which will be made obvious from the following detailed descriptions of the invention, with reference to the accompanying drawings, in which;

FIG. 1 is a longitudinal sectional View of a hollow cathode discharge tube embodying the present invention; and

FIG. 2 is an enlarged view of the hollow cathode of the discharge tube shown in FIG. l.

The principle of generation of Fraun'holfer lines has been applied for analysis of metallic elements in atomic absorption analysis. When a metallic vapor is in thermal equilibrium at a certain temperature, a portion of the metal atoms are in an excited state by heat energy, while most of the metal atoms are in ground state. If the abovementioned atomic vapor is irradiated by light continuously, and the transmitted light is examined, it is seen that light of wave-length corresponding to the energy level lof the atoms is absorbed by the atoms under the ground state. The amount of light absorbed in this case is in a definite relation with the amo-unt of metal atoms in the vapor, and quantitative analysis of the metal atoms contained in an unknown specimen can be effected by measurement of the rate of light absorption.

In practical atomic absorption analysis, resonance lines are mainly used as the irradiation light, as they are of great absorption strength. However, in general, the wave-length width of resonance lines is affected-'by minute construction of the spectrum, natural width, doppler width, pressure width, resonance effect, Stark effect, selfabsorption, etc. Consequently, a high-precision analysis cannot -be expected .unless resonance lines of narrow wave-length width are employed.

Among the above-enumerated factors affecting the wave-length width, the most important factors are Doppler width and self-absorption. Doppler width, caused by the thermal movement of luminescent atoms, a hollow cathode discharge tu-be is used which is operative in the range near abnormal glow discharging at low temperatures.

Conventional hollow cathode discharge tubes heretobefore generally used comprise cathodes of hollow cylindrical shapes constituted by elements to Ibe analysed, but in case of low-melting-point metals, particularly alkali metals, they Aare so readily oxidized and ignited that it is diicult to make the hollow cathodes with such metals.

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In order to solve the above problem, we tried to use hollow cathodes formed by alloys containing alkali metals, or to place alkali metal vapors in the inner-spaces of discharge tubes comprising hollow cathodes formed by other elements of high melting points. However, in the former case, there being limits to the containable amounts of alkali metals, problems exist with respect to strength of light land useful life of the tubes. In the latter case, undesirable discharges are apt to take place between anodes and outer peripheral portions of hollow cathodes, resulting in unstable discharges in general.

By reason of the above, discharge lamps having hot emission have been necessarily used in the case of alkali metals.l For atomic absorption analysis, thereV is required an electrical source for the above-mentioned hot emission discharge lamp, in addition to the electrical source for the hollow cathode discharge tube of usual construction adapted for other metals in general. However, because hot emission discharge lamps have great self-absorption good analytical precision cannot be obtained.

In order to eliminate the above problems, the present invention contemplates alkali metals held by the hollow cathodes by special construction. Thus, according to the present invention, the outer peripheral portion of the hollow cathode is formed of a substance through which any low-melting-point metal does not easily diifuse and permeate. The inner peripheral portion of the cathode is formed by a refractory and electro-conductive sulbstance which allows dii-Fusion and permeation therethrough of the loW-melting-point metals. The low-melting-point metals are Iaccommodated within a chamber formed in the cathode between the outer and inner peripheral portions.

Referring now to the drawings, the glass container 1 of the discharge tube is lled with argon or neon gas. The tube comprises, within the container 1, an anode 2, a hollow cathode 3 having a lead wire 4 connected thereto, and a steatite plate 5 for preventing discharge. The glass container or bulb 1 is provided with a window 6 for resonance line output. Referring particularly to FIG. 2, lthe :hollow cathode 3 comprises an outer peripheral portion 7 formed by iron, for example, and an inner peripheral portion 8 mo-ulded by powder metallurgy of refractory metal, such as for example, Ni, W, Mo, etc., and having about 50% porosity. The inner peripheral portion 8 is formed with an annular recess 9 outside to provide a chamber in which alkali metal, such as sodium, potassium, etc., is accommodated. The hollow cathode is lixed to the support by a screw 10.

When a suitable voltage is applied to a hollow cathode discharge tube as above-mentioned, hollow cathode discharge is caused to take place and sustained between the anode and hollow cathode through neon or argon gas in the bulb, to heat the cathode. By virtue of heating of the cathode, the alkali metal in annular recess 9 is melted or vaporized, and diffused and permeated through the porous inner peripheral portion 8, to appear thereinside and generate resonance lines of the alkali metal. Thereafter, the diffusion and permeation of the alkali metal through the inner peripheral portion 8 continue, generating alkali-metal resonance lines. Since the outer peripheral portion 7 of hollow cathode 3 is relatively nonporous and non-permeable for the alkali metal the alkali metal never appears on the outer periphery of the cathode, and there is no fear of occurrence of unnecessary or undesirable discharge.

It may readily be understood that t-he lowameltingpoint metals, such as sodium, potassium, etc., may be contained in the pores of the inner portion V8 itself of the cathode, instead of being accommodated in the annular Besides alkali metals, indium, lead, etc., are also included in the low-melting-point metals under consideration.

It should also be n-oted that the outer and inner peripheral portions of the `hollow cathode may respectively be formed by suitable materials other than metals, such as graphite. The inner peripheral portion 8 is not necessarilymoulded by powder metallurgy, but may be formed by any other porous material adapted for the purpose.

. As has been understood from the foregoing descriptions, the presentinvention has eliminated important faults of conventional hollow cathode discharge tubes for atomic absorption analysis of alkali metals and other low-meltingpoint metals, enabling practical manufacture of hollow cathode discharge tubes for use in atomic absorption analysis of low-melting-point metals. In addition, the

hollow cathode discharge tubes of the present invention are easy to manufacture, with stable operation and long useful life. Further, in atomic absorption analysis, any additional electric source is not necessary for alkali metals or other low-melting-point metals, resulting in simple construction and low cost. Atomic absorption analysis of low-melting-point metals are effected with a high precision, thus broadening the iield of application of atomic absorption analysis to the great practical advantage of the art.

We claim as our invention:

1. A hollow cathode discharge tube having a hollow cathode including chamber means within said hollow cathode for accommodating a low-melting-point metal, said hollow cathode comprising an outer peripheral portion formed of a substance which is resistant to the diff-usion and permeation of sai-d loW-melting-po-int metal, an inner peripheral portion vformed of a refractory and electro-conductive substance which allows diffusion and permeation therethrough of said low-melting-point met-al, and

said chamber means being formed between said outer and inner peripheral portions.

2. The hollow cathode discharge tube according to claim 1, in which said inner peripheral portion of the cathode includes a molded body formed by powder metallurgy of a refractory and electro-conductive substance.

3. A hollow cathode discharge tube having a hollow cathode, said hollow cathode comprising an outer peripheral portion, and an inner peripheral portion formed by a refractory and electro-conductive substance molded by powder metallurgy, said inner peripheral portion having pores for holding a low-melting-point metal, and said outer peripheral portion being formed of a substance which is difficult for said low-melting-point material to diffuse and permeate therethrough.

4. The hollow cathode discharge tube of claim 1 wherein said substance forming said inner peripheral portion comprises an element selected from the group consisting of Ni, W, Mo and C.

5. The hollow cathode discharge tube of claim 1, where-in said l-oW-melting-point metal i-s selected from the group consisting of In and Pb.

6. The hollow cathode discharge tube of claim 1, wherein said low-melting-point metal is an alkaline-earth metal.

References Cited by the Examiner UNITED STATES PATENTS 2,677,778 5/ 1954 Baker et al. 313-346 2,975,320 3/1961 Knauer 313-34 3,089,054 5/1963 Waish et al. 313-217 X 3,183,393 5/1965 Paterson 313-218 FOREIGN PATENTS 757,066 9/ 1956 Great Britain. 977,545 12/ 1961 Great Britain.

JOHN W. HUCKERT, Primary Examiner.

A. J. JAMES, Assistant Examiner. 

1. A HOLLOW CATHODE DISCHARGE TUBE HAVING A HOLLOW CATHODE INCLUDING CHAMBER MEANS WITHIN SAID HOLLOW CATHODE FOR ACCOMMODATING A LOW-MELTING-POINT METAL, SAID HOLLOW CATHODE COMPRISING AN OUTER PERIPHERAL PORTION FORMED OF A SUBSTANCE WHICH IS RESISTANT TO THE DIFFUSION AND PERMEATION OF SAID LOW-MELTING-POINT METAL, AN INNER PERIPHERAL PORTION FORMED OF A REFRACTORY AND ELECTRO-CONDUCTIVE SUBSTANCE WHICH ALLOWS DIFFUSION AND PERMEATION THERETHROUGH OF SAID LOW-MELTING-POINT METAL, AND SAID CHAMBER MEANS BEING FORMED BETWEEN SAID OUTER AND INNER PERIPHERAL PORTIONS. 